Stock-based Compensation Expense Fixed-Price Stock Options (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Shares
Outstanding Shares,Beginning Balance (in shares)	1,542,244
Granted (in shares)	82,204	141,661	313,803
Exercised (in shares)	(210,432)
Forfeited (in shares)	(12,298)
Outstanding Shares, Ending Balance (in shares)	1,401,718	1,542,244
Exercisable, Ending Balance (in shares)	768,436
Weighted-Average Exercise Price
Outstanding Weighted-Average Exercise Price, Beginning Balance (in dollars per share)	$ 51.85
Granted (in dollars per share)	92.93
Exercised (in dollars per share)	39.15
Forfeited (in dollars per share)	59.05
Outstanding Weighted-Average Exercise (in dollars per share)Price, Ending Balance	56.10	$ 51.85
Exercisable Weighted-Average Exercise Price, Ending Balance (in dollars per share)	$ 50.35
Weighted-Average Remaining Contractual Term (Years)
Weighted-Averaged Remaining Contractual Term, Beginning Balance	5 years 1 month	5 years 6 months
Weighted-Averaged Remaining Contractual Term, Ending Balance	5 years 1 month	5 years 6 months
Exercisable Weighted-Average Remaining Contractual Term, Ending Balance	3 years 3 months
Aggregate Intrinsic Value
Outstanding Aggregate Intrinsic Value, Beginning Balance	$ 52,798
Outstanding Aggregate Intrinsic Value, Ending Balance	100,632	$ 52,798
Exercisable Aggregate Intrinsic Value, Ending Balance	$ 59,585